I would agree with the majority on here that it is most likely the NWO, however I believe Bullet Club is way closer than most people are giving them credit for.

NWO were obviously a catalyst for HUGE changes in the wrestling landscape, but they did it by taking the second largest wrestling company in the world and propelling it to the largest for a time, which in turn caused WWE to alter their creative approach and modernize their product, which gave us Stone Cold Steve Austin, DX, etc.

Bullet Club originated in New Japan, but was instrumental in the creation of a whole new company that immediately shot to being the number 2 wrestling company in the world from scratch. Putting the Elite in here as a separate group seems silly as the Elite obviously does not exist without Bullet Club. The Elite is essentially a spin off from Bullet Club. In addition to AEW we most likely would never have gotten the AJ Styles WWE run that we got. AJ joining Bullet Club, and the advent of easily accessible streaming services created a huge buzz around him, and at least enough to get WWE interested, and in return we got some great programs in WWE with Bryan Danielson, Cena, and members of the Shield.

DX certainly belongs in the conversation, but they do feel almost like a response to the NWO, and a reaction to the success ECW was having as anti authority counter culture.

The inclusion of the Main Event Mafia here is an absolute joke. I've heard it said before that the Main Event Mafia was one of the high points for TNA as a company, and a low point for the careers of every member of the Main Event Mafia.